Depends on how it infests hosts, I would think.
I consider the banishment of the Cornerhounds as a pretty big hint on how to stop Nemesis. You banish/trap one and you get the whole lot.
I always thought there was a limit because otherwise why wouldn't it just infest everyone? The threat would be enormous and everyone would have lost by now.
Thirteen seems most likely based on that number's significance in the series, and the fact that the pack of Cornerhounds numbered at thirteen.
Thirteen is, to me, the second most likely number, and it could very well be correct. But as for finding one host and banishing Beside completely with just one seems... Optimistic? Certainly too easy for Harry, to say the least. And too easy for no-one to have managed it in all the years the Sidhe and Co. have been fighting "Nemesis". But who's to say it's even a possibility?
Probably needs it's own thread, but.. what's the quantifiable different between what Fearbringer does and what Nemesis does? Are other walkers also multifaceted beings or just nemesis? And... Has anyone considered the three beer bottles touching has nothing to do with the number of walkers, but the number of manifestations available? Three touching bottles is something I think is oftentimes skimmed over for the greater theoretics.(and of course, the vast assumption it means three walkers are coming, I've not seen challenged by anyone but me. That's the ambiguity of not actually saying anything..) Not three bottles standing together, three touching, as though intimately entwined somehow, interconnected.
I want to say it's been confirmed by WoJ, or stated in a way that seemed to confirm, there are three Walkers. And there might have been something about them being three parts of a whole, similar to what you're describing. But it's been too long for me to remember if the was just speculation or theory.
As for roles, I think we've seen what each is good at, and their limitations.
Before is leading the charge (from the Fore-front
) and willing to fight, but not all that great at it. He can debuff, and probably buff his own team, but he wasn't all that impressive in the bar fight until he whammied.
Beside is the spymaster who can manipulate the outcome of battles from the Side-lines (I crack myself up
) but he's even worse as a combatant.
He's struggled to control his hosts efficiently multiple times (Lea defied him, and he couldn't use Sith to his fullest combat capabilities).
Behind is the most potent, as stated in the books, because he's not stuck beHind (okay that one wasn't as good) some weak or resistant host. He's the assassin in the group that gets physical, but also got banished by a (starborn) kid because he underestimated his enemy.
So, in summary, there's the Cult Leader (Before), the Mastermind (Beside), and the Soldier/Assassin (Behind).